1. Technical Field
The subject invention relates to photographic processing compositions, film units, and processes for use in photographic diffusion transfer systems. More particularly, the subject invention relates to products and processes utilizing aqueous alkaline processing compositions including thickening agents therein.
2. Description of the Related Art
Diffusion transfer photographic products and processes have been described in the art and details relating to such products and processes can be found in numerous references, e.g. U.S. Pat. Nos. 2,983,606; 3,415,644; 3,594,165; 3,647,437; Great Britain Pat. No. 1,220,524; and chapter 6 of Imaging Processes and Materials, Neblette's Eight Edition, Van Nostrand Reinhold, N.Y., 1989, entitled "Instant Photography Related Reprograph Processes." In general, diffusion transfer photographic products include film units having a photosensitive system including at least one silver halide emulsion layer usually integrated with an image-providing material. After photoexposure, the photosensitive system is developed to establish an imagewise distribution of a diffusible image-providing material, at least a portion of which is transferred by diffusion to an image-receiving layer capable of mordanting or otherwise fixing the transferred image-providing material. In some diffusion transfer products, the transfer image is viewed by reflection after separation of an image-receiving element from the photosensitive system. Such products are often referred to as "peel-apart" film units. In other diffusion transfer products, such separation is not required and the transfer image is viewed against a reflecting background, usually provided by a dispersion of a white, light-reflecting pigment such as titanium dioxide. Such products are often referred to as "integral negative-positive" film units or simply "integral" film units.
A number of integral photographic products and processes have been proposed for providing diffusion transfer photographic images retained as part of a permanent laminate. Typically, such a photographic laminate will comprise a pair of outer polymeric layers, e.g., polyester, having therebetween a developed photosensitive system and a suitable image-carrying layer. In such products, the image-carrying layer is separated from the developed silver halide photosensitive system by a light-reflecting layer (e.g., a layer containing titanium dioxide positioned between the developed silver halide emulsion system and the image-carrying layer) and at least one of the outer polymeric layers is transparent to permit viewing of the photographic image against the light-reflecting layer.
In the processing of certain preferred integral diffusion transfer film units, it has been customary to effect photographic development with the aid of an aqueous alkaline processing composition including a light-reflecting agent, e.g. titanium dioxide. Such a processing composition is typically spread from a rupturable container by passage of the film unit between a pair of rollers to distribute a layer of processing composition between the sheet elements of the film unit. The distributed layer of processing composition forms a light-reflecting layer which provides a background for viewing of the photographic image and upon drying, may also serve to bond the layers of the film unit together to form the desired permanent laminate. Photographic processing compositions suited to such purposes are described, for example, in U.S. Pat. Nos. 3,415,644; 3,597,197; 3,679,409; 3,647,437; 4,202,694; 4,294,907; 4,324,853; 4,397,996; 4,496,651; and 4,680,247. As is pointed out in these patents, a thickening agent, (also referred to as a viscosity-increasing reagent/agent or viscosity-imparting reagent/agent), is usually included in such processing compositions to facilitate uniform spreading of the composition, e.g., a polymeric film-forming material such as sodium carboxymethyl cellulose, hydroxyethyl carboxymethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, polymeric oximes such as polydiacetone acrylamide oxime, and various blends thereof.
It is important that the thickening agents remain stable, that is, not hydrolyze, in the typically high pH of the processing composition. Furthermore, thickening agents must have stable shelf lives and absorb as little light as possible, thereby not significantly contributing to the minimum optical density (D.sub.min) of the film unit. With these concerns in mind, polyacrylic acid thickening agents, such as Carbopol 940.TM. (available from B. F. Goodrich, Specialty Polymers and Chemicals Division), have been used as thickening agents in processing composition resulting in improved shelf lives and reduced minimum optical density values.
A continual concern with processing compositions is phase separation of the constituents of the processing composition; that is, a condition wherein heterogenous layers of the constituents of the processing composition form, e.g. between a light-reflecting pigment-rich phase and other constituents of the composition. Typically such a phase separation forms in the rupturable container and is referred to as "pooling." This separation of constituents manifests itself when the processing composition is spread between the sheet elements of the film, and may result in an inconsistently developed photograph which includes portions having inaccurate image and color formation due to non-uniform development.
Efforts to eliminate pooling of the processing composition can be difficult without detrimentally affecting photographic quality of a film unit. That is, due to complexity of the chemical interactions between the constituents of the composition, minor changes in the formulation of the processing composition can have a significant impact on the resulting photographic product.